Electromagnetic radiation method for guiding the drilling of oil wells after the borehole has entered a massive earth formation of chemically deposited material, by a mistake, accident, or the like



412 815 ILLING OF OIL WELLS AFTER THE BOREHOLE HAS ENTERED A MASSIVEEARTH BYA N 6. 1968 w. T. HOLSER ETAL ELECTROMAGNETIC RADIATION METHODFOR GUIDING THE DR FORMATION OF CHEMICALLY DEPOSITED MATERIAL MISTAKE,ACCIDENT, OR THE LIKE 4 Sheets-Sheet 1 Filed Nov. 14, 1966 F|G.1A

INVENTORS WILL/AM r. HOLSER ROBERT R. UNTERBERGER STANLEY B. JONESATTORN Y5 Nov. 26, 1968 w r. HOLSER ETAL 3,412,815

ELECTROMAGNETIC RADIA'IION METHOD FOR GUIDING THE DRILLING OF OIL WELLSAFTER THE BOREHOLE HAS ENTERED A MASSIVE EARTH FORMATION OF CHEMICALLYDEPOSITED MATERIAL, BY A MISTAKE. ACCIDENT OR THE LIKE 4 Sheets-Sheet 2Filed Nov. 1966 K18 lololzlvlzk O 2 @EHEB /2 \\%$&& v v, 4/ w //\A\\' 1A J L=T l I I L Y=I I J l I l I r A AA A A A A A A I A A A A A A I: A AI: A A

A A A A A A A A A A A A A I\ A A A A A A A A A l A A A A A A q A AA A AA 2 A A A A I\ A E A A A A A A i 4Z l I I I l A A I A A A A 7 I l l filA n A AA fiI=l=l=l=i=l=l=l\' A AA A A A I I I =j I '.l-:,'- .7,

INVENTORS WILL/AM T. HOLSER ROBERT R. UNTERBERGER STANLEY B. .JONES-ATTORNEYS Nov. 26, 1968 w. T. HOLSER ETAL 3,412,815

ELECTROMAGNETIC RADIATION METHOD FOR GUIDING THE DRILLING OF OI ELLSAFTER THE BOREHOLE HAS ENTERED A MASSIVE EARTH FORMATION OF CHEMICALLYDEPOSITED MATERIAL, BY A MISTAKE, ACCIDENT, OR THE LIKE Filed Nov. 14,1966 4 Sheets-Sheet 3 0 TIME A I AVG O W Y Y Y- TIME E 7 2' POWER ---7SUPPLY SWEEP 5s GENERATOR L ISOLATOR I 58 OSCILLATOR 54 ATTENUATOR 626.3 BALANCED 59 I MIXER DETECTOR/6,

ATTENUATOR G---- as AMPLIFIER DISTANCE V FREQUENCY E METER INVENTORS AZT WILL/AM r. HOLSER H 9 /28 ROBERT R. UNTERBERGER 27 STANLEY 5. JONESFIG.

DEPTH United States Patent 3,412,815- ELECTROMAGNETIC RADIATION METHODFOR GUIDING THE DRILLING OF OIL WELLS AFTER THE BOREHOLE HAS ENTERED AMASSIVE EARTH FORMATION OF CHEMICALLY DEPOS- ITED MATERIAL, BY AMISTAKE, ACCIDENT, OR THE LIKE William T. Holser, La Habra, Robert R.Unterberger,

Fullerton, and Stanley B. Jones, Whittier, Calif., assignors to ChevronResearch Company, San Francisco, Calif., a corporation of DelawareContinuation-impart of application Ser. No. 253,339, Jan. 23, 1963. Thisapplication Nov. 14, 1966, Ser. No. 594,077

13 Claims. (Cl. 17541) ABSTRACT OF THE DISCLOSURE A method for aiding adriller-operator to redirect a borehole that has, by mistake, accident,or the like, entered a first massive rock formation of chemicallydeposited materials, such as a salt dome, into contact with a secondearth formation having oil reservoirs in locational association with theinterface formed between the first and second formations. Thefirst-mentioned formation is irradiated with electromagnetic energy froman electromagnetic generator in the borehole. By measuring the two-waytravel time of the irradiated energy with respect to the time ofreception of reflected energy from the interface, the distance to theinterface from the borehole is determined, whether the interface liesbelow or at a lateral distance from the borehole. With this knowledge,the operator redirects the well bore to encounter the second earthformation and its associated oil reservoirs.

This application is a continuation-in-part of application Ser. No.253,339, filed Jan. 23, 1963.

The present invention relates to drilling oil wells. More particularly,it relates to a method for aiding a drilleroperator to redirect aborehole that has, by mistake, accident, or the like, entered a massiverock formation of chemically deposited materials, such as deposits ofrock salt (halite), anhydrite, limestone or the like, into contact withoil reservoirs adjacent to the side or bottom of the borehole. By theterm chemically deposited materials, it is meant not only to includematerial such as rock salt (halite) and anhydrite formed byprecipitation, but,

also to include all types of limestones including those in which thechemical deposition has included biological processes as Well as thoselimestones precipitated inorganically.

In accordance with the present invention, the extent of sidetracking ofa well bore :in order to encounter the oil reservoirs associated withthese types of formations is guided by information obtained fromirradiating the formation with electromagnetic radiation to measure thedistance from the borehole to the interface of the formation, whetherthe interface lies below or at a lateral distance from the well boreand, with this knowledge, the operator redirects the well bore toencounter the oil reservoirs.

In exploration for oil and gas, it is known that commericalaccumulations of oil and gas may be present in reservoirs associatedwith massive rock formations of chemically deposited materials, such asrock salt, anhydrite, limestone, or the like. For example, in thesouthcentral section of the United States, commercial accumulations ofoil are usually present, if at all, in the sedimentary formationsdirectly adjacent to a salt dome. It is generally believed that theseaccumulations occur because the sedimentary beds are uptilted by theupward intru- Patented Nov. 26, 1968 sion of the salt dome into thesedimentary layers. The upward tilt of the beds at the interface createsa pocket, or trap, where oil and gas can accumulate by gravityseparation from other formation fluids.

Similarly, in exploring for gas and oil in earth formations that wereonce below sea level, it is known that oil may be present in porouslimestone reefs formed at the edge of marine basins. These reefs oncerose'steeply from the oceans bottoms and then were buried by laterdeposited materials. Recent geological evidence indicates that thissedimentary material about reefs is not necessarily the type ofimpervious formations usually associated with sedimentary oil traps,such as shale, but may be formed of chemically deposited materials, suchas rock salt, anhydrite, or dense limestone. These chemically depositedmaterials have extremely small pore space so that oil or gas can betrapped within the reef and accumulate by gravity separation from theformation fluids.

While the general location and form of a salt dome or a reef surroundedby these chemically deposited materials may be found at the earthssurface by gravity or seismic prospecting techniques, it has been foundthat the exact location of the oil reservoirs associated with thesestructures can often only be determined by actually drilling a well.This is because the shapes of the salt doom or reefoften irregular bythe depths of interest-are not clearly defined by the surface seismicdata. Even when actually drilling, should the borehole enter, byaccident or the like, the salt dome or the rock formations surroundingthe reef, there is still considerable doubt in the operators mind as tothe proper direction the well bore should take to encounter the oilreservoir. For example, if he has drilled into a salt dome, he mustconsider whether he should continue drilling along the same direction ofthe well bore, assuming the well bore is in a salt ledge or overhang, orWhether he should whipstock away from the center of the dome toward theinterface of the sedimentary beds and the salt dome, assuming the domeis shaped more like a pyramid. Accordingly, it is the primary purpose ofthis invention to accurately delineate from a borehole the position ofan interface of the massive rock formation of chemically depositedmaterial adjacent to an oil-bearing structure or reservoir after thewell bore has entered chemically deposited rock formations by mistake,accident, or the like, so that the well bore can then be redirected withenough accuracy to penetrate the reservoir.

In a previous patent application (Method for Mapping a Salt Dome atDepth by Measuring the Travel Time of Electromagnetic Energy EmittedFrom a Borehole Drilled Within the Salt Dome, William T. Holser, RobertR. Unterberger and Stanley B. Jones, US. Patent No. 3,286,163 issuedNov. 15, 1966) we proposed to use electromagnetic radiation from aborehole deliberately drilled into a salt dome for mapping its sidewalls to accurately guide the subsequent drilling of the wells topenetrate the sedimentary beds adjacent the side walls of the dome. Inthe present method, we extend the use of the electromag netic rangingtechnique for use in massive rock formations of chemically depositedmaterials, including salt domes and the chemically deposited rockformations surrounding reefs, to guide the redirection of the well boreto encounter oil reservoirs associated with these types of structuresafter the well bore has, by accident, mistake, or the like, entered suchmassive rock formations.

Further objects and advantages of the invention will become apparentfrom the following detailed description, taken in conjunction With theaccompanying drawings, which form a part of this specification.

In the drawings:

FIGURE 1 is a sectional view of a borehole penetrating a salt dome andillustrates the position of an electromagnetic energy transmitting andreceiving sonde within the well bore along with associated equipment atthe earths surface to detect and record the distance to the interfacebetween the salt dome and the oil-bearing sedimentary formations, bothto the side and below the borehole, after the borehole has entered thesalt dome by accident, mistake, or the like;

FIGURE 1A illustrates in greater detail a transmitting and receivingantenna useful in the system of FIGURE 1;

FIGURE 2 is a sectional view of a well bore penetrat ing a chemicallydeposited rock formation formed about a reef containing oil and alsoillustrates the position of an electromagnetic energy transmitting andreceiving sonde within the well bore along with associated equipment atthe earths surface for measuring and detecting the distance to the reeffrom the well bore, both to the side and below the borehole;

FIGURE 3 is a waveform diagram useful in understanding a frequencymodulating radiating system for ranging to an interface where theinterface to be mapped from the well bore is close to the well bore;

FIGURE 4 is a schematic diagram of a transmitterreceiver and associatedcircuitry for determining lateral distance in the frequency modulatingranging system;

FIGURE 5 illustrates an alternative antenna system useful in mapping aninterface located below the bottom of the well bore;

FIGURE 6 illustrates in greater detail a logging sonde including anelectromagnetic pulsed antenna system useful in determining the lateraldistance to an interface from a borehole in accordance with the methodof the present invention.

Reference is now made to the drawings. In particular, FIGURE 1schematically indicates use of a method of this invention to map, atdepth, the location of the wall of the salt dome 10 after Well bore 11has entered the salt dome by mistake, accident, or the like. The purposeof such mapping is to aid the operator in determining the direction thewell bore should take to encounter sedimentary beds 12, 13 and 14adjacent to the salt dome. As shown, these beds are normally tiltedupward by the intrusion of the salt dome 10 through the beds after theyhave been laid down horizontally. The salt dome in such cases may beformed to have an overhang 15 so that, after the well bore haspenetrated the overhang, there is considerable doubt as to the directionthe well bore should take to encounter oil-bearing sedimentary beds 12and 14. (The initial location and direction of well bore 11 to encounterbeds 12, 13 and 14 are based on data obtained from top-surface seismicoperations, often inconclusive at the depths of interest.) Obviously, ifthe exact horizontal and vertical locations of the wall of the dome areknown-Le, to the side and directly below the well borethe operator canredirect the drilling of the well in the most economical manner toencounter the oil reservoirs 16 and 17. In some cases, the mosteconomical manner may be to abandon the borehole and drill a new well ata new location on the earths surface. In this regard, the costs alreadyexpended in drilling the borehole versus the costs of developing a newwell at a different site at the earths surface are used for thecomparison. Among the factors which affect the decision are depth ofpresent well, distance to the interface, extent and location of causingin place, ease of sidetracking the well, etc.

FIGURE 2 schematically illustrates the use of the method of the presentinvention in another application. As shown, a borehole 20 penetrates anearth formation 21 formed of a chemically deposited material-such asrock salt.(halite), anhydrite, or dense limestoneadjacent to a reef 22,say of limestone containing oil reservoirs (not shown). The purpose ofborehole 20 is to penetrate reef 22 to develop such reservoirs. The reef22, together with the surrounding formation 21, being formed ofimpervious material forms traps for gravity accumulation of the oil fromother formation fluids associated with the forma- 4 tion of reef 22.When the borehole has entered the adjacent formation 21 by accident,mistake, or the like, there may be considerable doubt in the drillersmind as to the proper direction the borehole should take to penetratethe reef 22 for the same reason previously mentioned. The method of thepresent invention is useful in mapping, at depth, the horizontal andvertical location of the interface of the reef 22, both to the side andbelow the borehole, to guide the driller in selecting a direction forthe borehole 20 that will penetrate the reef and allow development ofits oil. If the exact horizontal or vertical location of the interfaceof the reef 22 is known, the borehole can be redirected by the drillerin the most economical manner to encounter the reef.

In carrying out the purpose of the present invention, the logging sonde24 is supported by cable 25 within the well bore penetrating theformation of interest, for example, well bore 11 within salt dome 10(FIGURE 1), to map the horizontal and vertical location of the interfaceof the salt dome, or as in another example, within borehole 20 of aformation 21 to map the interface of reef 22 (FIGURE 2). The distance tothe interface of the salt dome or to the reef from the borehole isdetermined by transmitting pulsed or frequency modulated electromagneticenergy through the adjacent formation surrounding the borehole anddetecting the portion of the energy reflected from the interface. Bymeasuring the time between transmission and reception of theelectromagnetic energy as measured by analysis of outgoing and incomingpulses or analysis of their differences in frequencies, the interface ofdome 10 or reef 22 can be indicated and displayed at the earths surfaceinasmuch as the velocity of the energy in the formation is known.

In carrying out the method of the present invention, the electromagneticenergy has a frequency in the range of at least 10 hertz but not greaterthan 10 hertz so as to better propagate within the adjacent formationsurrounding the borehole without undue dispersion or attenuation. It hasbeen found in loss-tangent measurements in samples of halite taken fromactual salt domes that the method of this invention operates withmaximum efficiency at frequencies within the aforementioned frequencyrange. It is also known in crystallization of salt to form a salt domeor in the formation of sedimentary beds of halite, anhydrite, or denselimestone about a reef that frequently small pockets of the originalbrine are left. These pockets of saturated salt water will have adimension of a few millimeters but seldom include large amounts or largepockets because of the small pore size of these formations. Accordingly,electromagnetic waves will travel through these relatively homogeneousformations and return by reflection from the interface remote from theborehole without undue attenuation or dispersion of the waves.

Surface-recording equipment for indicating the distance to the interfaceof the salt dome or the reef is indicated at 18 in FIGURES 1 and 2 andincludes three indicators: for depth, 26; for distance, 27; and forazimuth, 28. Depth indicator 26 shows the mapping depth of the sonde 24within boreholes 11 and 20. The mapping depth is measured by pulley 29;in turn, the pulley 29 is shown on indicator 26. The distance from theborehole to the mapped interface at the mapping depth is indicated bythe time between transmission and reception of the electromagneticenergy at the sonde 24 and the known velocity of the energy in theformation. The time may be indicated in two Ways: by analyzing the timeelapsed between the emission and reception of pulses of the energy, orby determining the differences in frequency of the transmitted andreceived energy as the output frequency is varied. Azimuthal directionof the radiated energy, if directive emission is used, may be indicatedby position indicator 28, here shown as an oscilloscope. By physicallyassociating depth indicator 26, distance indicator 27 and azimuthalindicator 28, the information on all three units can be assimilated toindicate the distance and direction of the mapped interface relative tothe borehole whether to the side or below the logging sonde. With thisinformation, the driller redirects the well bore in the most economicalmanner to encounter oil reservoirs associated with salt domes and reefs.

In mapping an interface, in accordance with the present invention, thesonde 24 is preferably held stationary at a location closely adjacent toor in contact with the bottom wall 23 of the borehole. The preferredmapping depth will thus allow the operator to take greatest advantage ofthe already drilled extent of the borehole. Antennas within the sonde atthe preferred mapping depth may also have an azimuthally omnidirectionalradiation pattern, say as provided by a dipole antenna. On suchapplications, the first received signal at the distance indicator 27represents the nearest interface of the mapped salt dome or reef. Theazimuthal direction of the interface may be approximated with referenceto surface sonic data.

Where the surface sonic data is inconclusive, however, it may bedesirable to utilize antennas having a more directive antenna pattern inazimuth as in FIG. 1A. As indicated, transmitting and receiving antennas30 and 31 are illustrated as supported within housing 32 on bearings 33having their ends flared as illustrated. They are called horn antennasand can be dielectrically loaded to reduce their dimensions at thefrequencies of interest. These antennas are most useful when theirprincipal axes of radiation are substantially normal to the interface ofthe salt dome or reef to be mapped. Accordingly, since the location ofsuch interfaces vary, in azimuth, relative to the well bore, an antennarotor 35 may be connected to the antennas through gears 36 and 37 fortheir controlled rotation about the axis of the well bore. The rotor 35includes a sensor suitably connected by leads forming a portion of thecable 25 to the indicator 28 at the earths surface for azimuthallyindicating the direction of the launched and received energy. Therotation of the rotor 35 is initiated by associated circuitry Within thesonde and at the earths surface well known in the control art.

FIGURE 5 illustrates an alternative directive antenna system withinsonde 24 useful in directing electromagnetic energy in a downwarddirection through bottom wall 23 of the well bore. As indicated,electromagnetic energy in the aforementioned frequency range is emittedfrom a transmitting horn antenna 40 and can be either frequencymodulated or pulsed. Although the antenna 40 is illustrated asstationary in elevation relative to the sonde 30 so as to transmitenergy into the formation below the well bore in only a single downwarddirection, it can be provided with suitable mechanisms, such as gearsenergized by an antenna rotor through appropriate circuitry, to rotateit about an axis normal to that of the well bore. This may be desirablewhen the operator desires to have a more extensive picture of the widthof the salt dome or reef below the well bore. After the energy isreflected from the interface of the salt dome or the reef, the returningelectromagnetic energy is then detected at the sonde by receiving hornantenna 41 pointed in the same direction as transmitting antenna 40.

In mapping a segment of a salt dome from within a borehole penetratingthe salt dome or in mapping a reef from a borehole exterior of the reef,a borehole may be spaced a relatively short distance from the interfaceto be mapped, say from one inch to several hundreds of feet. It is,therefore, proposed in such cases that a frequency modulated (FM)ranging system, operating within the aforementioned frequency range, beemployed to measure these small distances.

FIGURE 3 illustrates a principle of operation of an FM ranging system.The transmitter of the FM system has a central frequency i equal to atleast 10 hertz but less than 10 hertz (cycles per second). The frequencyof the transmitter is varied from f to and f, as shown, in a linearfashion, but such that ;f+ is within the above frequency range. Thisvariation can be sinusoidal, however, as it can be shown that theaverage frequency difference over a cycle of sinusoidal modulation isequivalent to that obtained from a linear variation within the samemodulating period. One cycle of this variation is accomplished. at arate of f hertz so that the time required to vary the energy through thefull range of frequencies (one full cycle) is 1/ f seconds. In thelength of time that'it has taken to transmit energy out to the interfaceandfor that energy to be reflected back to the sonde, the frequency thenbeing transmitted by the transmitting antenna has changed in frequencyby a certain finite amount determined by the rate at which thetransmitter frequency is being varied.

In FIGURE 3, travel time of the wave is illustrated as a time delay andis represented by the quantity Zd/ v, where d is the distance to theinterface and v is the velocity of transmission of the energy throughthe transmitting formation and is given by v=c/n=c/ /E/E where c is thespeed of light and n and E'/E are the index of refraction and the realpart of the dielectric constant of the formation normalized by that offree space, respectively. The difference in frequency of the transmittedenergy and the reflected energy represents the distance to the interfaceand back; and, if these two signals are beat one against the other, in asuitable mixer, the resulting difference frequency may be employed todetermine the distance to the salt dome or reef. This determination isbased on a knowledge of the index of refraction of the interveningformation, as determined by an analysis of cores taken from thatformation during drilling of the well bore.

The relationship of the difference in frequency to distance is found inthe following equation:

Difference in frequency=rate of change of the changing frequencyxtimebetween transmission and the reflection Af -R X T which can be writtenas:

I E. where =modulation rate. B =band width of the frequency modulationd=lateral distance to the interface, and v=the velocity of transmissionin the formation which, for measurement purposes, is equal to:

v=c EL 0 where c=velocity of light in air E=the real part of the complexdielectric constant of the formation traversed by the energy at thefrequency center E =the real part of the complex dielectric constant offree space.

To improve the near range resolution of the system, the rate of changeof frequency (R can be increased by increasing the band width (B) offrequency modulation. In this regard, it has been found that the rate ofchange of the changing frequency (R can be equal to about 10 to 10 hertz(cycles per second) for interfaces spaced at distancev from a few inchesto much longer distances from the ranging system.

FIGURE 4 illustrates a schematic diagram of a ranging system forperforming the method of the present invention. In this figure, anoscillator 50 is energized by power supply 51 to generate the basicfrequency for transmission into the adjacent earth formation surroundingthe borehole. The oscillator may be a magnetron or klystron capable ofoperating at the desired frequencies and power output. A sweep generator52 is synchronized with the oscillator and generates a varying potentialat the frequency f to cause variation of the transmitted frequency aboutits center frequency f The output of the oscillator is supplied throughan isolator 53 to a transmission line 54 carrying the energy totransmitting antenna 55. Between the isolator 53 and the transmittingantenna is a directional coupler 58 for sampling the frequency of theoscillator 50. The sampled signal is supplied through attenuator 59 tobalanced mixer detector 61.

As shown in FIGURE 4, receiving antenna 62 is located adjacent to thetransmitting antenna 55 and connected through a transmission line 63 toan attenuator 64. The output of attenuator 64 is supplied as a secondinput to balanced mixer detector 61 where the transmitted and receivedsignals are mixed to develop a difference frequency. This differencefrequency is fed into amplifier 65. A frequency meter 66 measures thefrequency of the signal from the balanced mixer detector and suppliesthat information to the distance indicating device 27 at the earthssurface. A camera (not shown) can be utilized to photograph the distanceinformation on distance indicator 27 from which the distance to a saltdome or reef from the present location of the well bore can bedetermined. The given distance on indicator 27 is associated with thedepth on digital indicator 26 and the azimuthal information on indicator28.

Another form of the transmitter-receiver circuit for the FM rangingsystem of the present invention is shown in phantom line in FIGURE 4 andemploys a single antenna for both transmitting and receiving theelectromagnetic energy to reduce both the size of the downholecomponents of the equipment and the near range resolution of the system.In accordance with this embodiment of the invention, a single antenna,say antenna 55 of FIGURE 4, can be adapted for this purpose byconnecting a directional coupler 70 (shown in phantom line) in seriesbetween the antenna and attenuator 64 to supply the second input tobalanced mixer detector 61. A the mixer detector 61, the transmitted andreceived signals are then beat together to produce a differencefrequency into amplifier 65 and, ultimately, to give an indication ofdistance to the interface, as previously described.

Another modification of the system not shown herein is the use of a hornantenna with modifications to develop circularly polarizedelectromagnetic energy, such as placing quarter wave plates within thebody of the transmitting horn antenna. In situations where the presentinvention is useful, under certain conditions only circularly polarizedenergy can be transmitted successfully through formations having ratherhigh water content.

FIGURE 6 illustrates an alternative antenna system employing pulsedelectromagnetic energy. In this embodiment, the sonde includes aninstrument housing 80 which preferably includes a high-frequencytransmitter 81 and suitable coupling and timing circuits 82 to sendelectromagnetic pulses to slot antenna 83. The slot antenna 83 includesa cylindrical housing supported on bearings 84. The pulses ofelectromagnetic energy are radiated from the antenna in an almostomnidirectional, azimuthal pattern normal to the longitudinal axis ofthe antenna but, because of slot 92, have a cusp or null in oneazimuthal direction. The timing circuits 82 control the switching of theantenna 83 periodically from transmitter 81 to receiver 85 by means of aTR switch 86. The output of receiver 85 is transmitted to the earthssurface for indication of the travel time of the wave to and returningfrom the mapped interface of the salt dome or reef. Power supply 87 isshown positioned within the housing 80 but, of course, may be located atthe earths surface if permitted by the electrical characteristics ofcable 88. At the earths surface, surface-recording equipment includesindicators for depth, for azimuth, and for distance. The distance fromthe borehole to the nearest reflected side of the salt dome or reef isindicated by the two-way travel time of a pulse of energy and thevelocity of the energy in the formation. The azimuthal direction of theinterface is indicated by rotating the antenna by means of antenna rotor90 and gears 91 as the pulses of electromagnetic energy are transmittedand received. The first returning echo signal received at thetransmitter represents the closest interface of the salt dome or reef.The slot antenna 83 is rotated until the first returning echo disappearseither from an indicator within the sonde or at the earths surface. Byassociating the null in the returning signal with the azimuthaldirection of the slot 92 of the antenna, say by a sensor within therotor 90, the azimuthal direction of the near interface is determinedand displayed.

While certain preferred embodiments of the invention have beenspecifically disclosed, it should be understood that the invention isnot limited thereto as many variations will be readily apparent to thoseskilled in the art, and the invention is to be given its broadestpossible interpretation of the following claims.

We claim:

1. A method for guiding the drilling of a well bore for penetrating oilreservoirs of at least a first earth formation in locational associationwith an interface formed between said first earth formation and a secondrock formation of chemically deposited salt, limestone, anhydrite, orthe like, capable of passing electromagnetic energy without undueattenuation or dispersion, after said borehole has entered that saidsecond formation by accident mistake, or the like, comprising:positioning in said well bore within said penetrated second formation alogging sonde at a known logging depth including an electromagneticgenerator and an electromagnetic receiver, said generator having anoutput in a frequency range of 10 to 10 hertz; irradiating saidformation with electromagnetic energy from said electromagneticgenerator in said logging sonde toward said interface; detecting aportion of said irradiated electromagnetic energy that is reflected backfrom said interface to said receiver in said logging sonde; comparingthe time of travel of said transmitted and received energy to derivedata indicating the distance to said interface at said known loggingdepth; and thereafter directin the drilling of the well bore based onsaid data to encounter said first formation.

2. A method in accordance with claim 1 in which said electromagneticenergy is continuously emitted but its central frequency within saidfrequency range is varied between a frequency above and below saidcentral frequency and said distance to said interface is determined bycomparing the instantaneous frequency of the transmitted energy withthat of the received energy.

3. A method in accordance with claim 1 in which said electromagneticenergy is a pulse of radiation and said distance to said interface isdetermined by measuring the two-way travel time of said pulse betweensaid interface and back to said sonde.

4. A method in accordance with claim 1 in which said electromagneticenergy propagates omnidirectionally in a plane through said sonde andsaid distance to said interface is the nearest distance relative to saidwell bore in said plane of propagation.

5. A method in accordance with claim 1 in which said energy is directedinto a confined path and said distance is determined in accordance withthe lateral and azimuthal orientation of said path relative to said wellbore.

6. A method in accordance with claim 5 in which said path issubstantially parallel to the axis of the well bore and said energypasses through the bottom of said well bore.

7. A method in accordance with claim 5 in which said path issubstantially horizontal relative to said well bore and is of a knownazimuthal direction relative to said well bore. t

8. A method in accordance with claim 1 in which said energy is directedin azimuth in a modified omnidirectional pattern having a known nulldirection therein, and said distance to said interface is the closestdistance relative to said well bore in said azimuthal direction ofenergy propagation.

9. A method in accordance with claim 8 in which the azimuthal directionof said closest interface is determined by sequentially irradiating saidformation as said null direction is varied in azimuth, said azimuthaldirection being indicated when the null direction in azimuth coincideswith that of said closest interface.

10. A method in accordance with claim 1 in which said last-mentionedstep of directing the drilling of the well bore to encounter said firstearth formation includes sidetracking said well bore in a new directionrelative to its existing direction.

11. A method in accordance with claim 1 in which said last-mentionedstep of directing the drilling of the well bore to encounter said firstearth formation includes continuing drilling along the existingdirection of the well bore.

12. A method for guiding the drilling of a well bore for penetrating oilreservoirs in at least a sedimentary formation in locational associationwith an interface formed between said sedimentary formation and a saltdome after said borehole has entered said dome by accident, mistake, orthe like, comprisingz' positioning in said well bore within saidpenetrated dome a logging sonde at a known logging depth including anelectromagnetic energy generator and receiver, said generator having anoutput in a frequency range of 10 to 10 hertz; irradiating said domewith electromagnetic energy from said electromagnetic generator in saidlogging sonde toward said interface; detecting a portion of saidirradiated electromagnetic energy that is reflected back from aninterface of said dome to said receiver in said logging sonde; comparingthe time of travel of said transmitted and received energy to derivedata indicating the distance to said interface at said known loggingdepth; removing said logging sonde from said well bore; and directingthe drilling of the well bore based on said data to encounter saidsedimentary formation.

13. A method for guiding the drilling of a well bore for penetrating oilreservoirs associated with a reef within a rock formation formed oflimestone, anhydrite, or the like, after said borehole has entered thatsaid formation by accident, mistake, or the like, comprising:positioning in said well bore Within said penetrated formation a loggingsonde at a known logging depth includin an electromagnetic energygenerator and receiver, said generator having an output in a frequencyrange of 10 to 10 hertz, irradiating said formation with electromagneticenergy from said electromagnetic generator in said logging sonde;detecting a portion of said irradiated electromagnetic energy that isreflected back to said receiver in said logging sonde from an interfaceformed between said reef and said formation; comparing the time oftravel of said transmitted and received energy to derive data indicatingthe distance to said interface at said known logging depth; removingsaid logging sonde from said well bore; and directing the drilling ofthe well bore based on said data to encounter said reef.

References Cited UNITED STATES PATENTS 3,286,163 11/1916 Holser et. al324-6 3,350,634 10/1967 Hoehn 324-6 RUDOLPH V. ROLINEC, PrimaryExaminer.

GERARD R. STRECKER, Assistant Examiner.

